The present invention relates to optical imaging technology and, more particularly, to measurement of and/or electronic compensation of optical phase distortion induced by an aberrating medium.
Optical signals may be significantly distorted as they pass through an aberrating medium such as the atmosphere. The phenomenon presents severe difficulties in achieving desired resolution in high quality optical imaging systems. The distortion generally takes the form of an aberration which can be represented locally as a tilt or slope of the wavefront of the received optical signal. Corrective compensation requires that either the local tilts due to the aberrating medium be measured and appropriate corrections be applied in the imaging process, or that the imaging process itself be rendered insensitive to the aberrations.
Heretofore, systems of direct measurement of the aberrations had the disadvantage of requiting the use of incoherent or unresolved sources. None of the techniques could use a coherent, extended source as the basis for measurements. In addition, prior measurement techniques are not inherently self referencing and require a known optical source. Prior imaging techniques which are insensitive to aberrations require either non-deterministic image sharpness algorithms, or the use of unwieldy and complex optical phase shifting means.
One conventional prior method of direct aberration measurement uses a Hartmann-type wavefront sensor to measure the local tilts of the wavefront. A basic Hartmann sensor has a detector array of individual photosensitive cells that sense the optical signal passing through a portion of the system aperture. That portion constitutes a subaperture that is focused by a lenslet array onto individual detector cells. Under ideal circumstances, the lenslets focus the subaperture portion of the incoming optical signal squarely on the center of the individual detector cells. However, the lenslets are sensitive to local tilts in the incoming wave front, and focus the imperfect signal off the center of respective detectors according to the tilt. This off center measurement is the indication of the local wavefront tilt. The total wavefront aberration can then be reconstructed by combining and processing these local tilt measurements. A serious difficulty in implementing this type of wavefront sensor lies in the nature of the off center determination. This requires an extremely accurate and stable mechanical alignment of the optical elements, and/or calibration scheme employing a local reference source. The alignment and calibration schemes are difficult in a practically realizable system.
A conventional method of performing imaging measurements which are insensitive to the presence of aberrations uses three laser beams illuminating an object of interest. The sources of the beams are separated by an amount Dx in the x direction and Dy in the y direction. The three beams are directed at a target. A resulting complex field pattern is produced by the three beams prior to passing through the aberrating medium. The field patterns of each of the three beams are spatially displaced, but are otherwise identical. The beams are mutually coherent, and thus create an interference pattern in the plane of a detector array. The processing of data at the detector array containing the interference pattern is essentially identical to that employed in the "4-bin" shearing interferometer algorithm developed by ITEK Corporation. In this case, the four distinct phases needed for processing at the detector array are created by pulse train forming optics at the laser end of the system.
Thus this system is implemented as a shearing wavefront slope sensor, so that the local wavefront slopes of the pattern at the detector array is measured. Since a shearing interferometer is a common path interferometer, the measurements at the detector may are insensitive to, and independent of, the aberrating medium. However, when this system is used, it is difficult to directly measure the intervening aberration. Furthermore, the system requires a complex beam division and phase delay system in order to provide the four distinct phases described above in the transmitted illuminator pulses.
Accordingly, it is an objective of the present invention to provide for direct measurement of intervening aberration in an imaging optical system. It is a further objective of the present invention to provide a system that does not require unreasonably stringent mechanical alignment of the optical elements. It is also an objective of the present invention to provide a system wherein either mutually coherent or mutually incoherent laser beams may be used to provide an imaging system that compensates for intervening aberrations as image enhancing corrections.